The invention relates to a ceramic dielectric based on bismuth-containing BaTiO.sub.3. The invention further relates to the use of this ceramic dielectric in multilayer capacitors.
In the trend toward of the miniaturization of electronic circuits, the requirement for capacitors having small dimensions and large capacitances is becoming increasingly important. In principle the following, the capacitance can be increased by:
1. Increasing the area of the dielectric, PA1 2. Increasing the relative dielectric constant, .epsilon..sub.rel, and PA1 3. Reducing the thickness of the dielectric.
In order to increase the area of a dielectric, few possibilities are available. This is because the maximum possible area is fixed by a series of secondary conditions, for example production machines and size restrictions with respect to the subsequent use of the capacitors.
It is known to achieve an increase of the area of the dielectric without a substantial increase of the dimensions of the finished component via so-called multilayer capacitors, in which thin ceramic foils are stacked with metallic electrodes to form a laminated packet which is then sintered to form a finished component. With the multilayer construction capacitors can be miniaturized with simultaneous higher capacitance values.
A further increase of the capacitance values of capacitors with miniature dimensions is possible when dielectrics are used having comparatively high relative dielectric constants. It is generally known to achieve an increase of the relative dielectric constant, .epsilon..sub.rel, by using ferroelectric materials, for example mixed crystals of BaTiO.sub.3, in which values of .epsilon..sub.rel =2,000 to .epsilon..sub.rel =10,000 are known. With an increasing dielectric constant .epsilon..sub.rel, however, the materials become more and more unstable so that the dependence of the dielectric constant on temperature and field strength consequently deteriorates considerably. Since the occurrence of these instabilities is coupled directly with the .epsilon.-increase, the physically possible limits of these materials have already been reached.
Although BaTiO.sub.3 has a comparatively high relative dielectric constant, .epsilon..sub.rel, it is particularly difficult to construct multilayer capacitors from this material. The temperatures which are required for densely sintering BaTiO.sub.3 are so high that the electrodes of the multilayer capacitors, which are to be provided prior to sintering the ceramic dielectric, must consist of high-melting-point noble metals, for example palladium.
Experiments have been carried out to manufacture dielectrics for multilayer capacitors based on BaTiO.sub.3, in which the sintering temperature was reduced from a first range of 1300.degree. to 1350.degree. C. (for pure BaTiO.sub.3) to 1080.degree. to a second range of 1100.degree. C. by the addition of PbBi.sub.4 Ti.sub.4 O.sub.12 in quantities from 2.5 to 4.75 mol.% and an extra addition of 1 to 2.25 mol.% BaO.Nb.sub.2 O.sub.5, BaO.Ta.sub.2 O.sub.5, PbO.Nb.sub.2 O.sub.5 and/or PbO.Ta.sub.2 O.sub.5. With this known ceramic, dielectric constants of .epsilon..sub.rel =1200 to 1800 can be achieved. These dielectrics also exhibit a comparatively good temperature stability of the dielectric constant of .+-.17% over a temperature range from -50.degree. C. to +130.degree. C. (Compare, U.S. Pat. No. 3,619,744.)
With the trend toward increasing miniaturization of components for so-called hybrid circuits, it has proved desirable to improve the parameters of the known ceramic as much as possible. For example, it is desired to achieve a higher relative dielectric constant, without increasing the sintering temperature and without increasing the dependence of the dielectric constant on temperature and on field strength.